Silicon dioxide (oxide) is a dielectric material widely used in the fabrication of integrated semiconductor circuits. The oxide thickness determines whether the oxide prevents shorting (insulator), or induction of electrical charges on the wafer surface. When used to prevent electrical charge induction from the metal layers, the oxide is referred to as a field oxide (FOX) layer. The magnitude of the voltages in the integrated circuit impacts the thickness of the FOX regions. By example, in fabricating a memory product, the core of the die is used to fabricate memory circuit elements, while the periphery is used for logic circuitry. Memory circuits operate at, or below, the 5.0 Vdc range, while other circuitry, such as logic circuitry, operates in the 10 Vdc to 20 Vdc range. The higher voltages utilized in the periphery requires a thicker FOX than the FOX used in the core of the die(4000 .ANG. compared to 2000 .ANG.). In order to fabricate the die with the two thicknesses of FOX in the core and periphery areas of the die, a dual FOX layering process must be employed.
As is known in the prior art, masks are provided with an alignment mark for use in aligning the various patterns on the wafer. A first mask creates a target on the wafer at a first patterning step. Subsequent masks contain masks portions which align to the previously formed mask. In dual field oxide fabrication processes, the second masking operation has traditionally caused a second layer of oxide material to be fabricated over the previously fabricated alignment marker. The subsequent alignment after the dual field oxide process has caused misalignment problems and device failures. Any attempts to test for mask misalignment is frustrated because of the second layer of oxide material that has been fabricated over the previously fabricated alignment marker. Thus a need is seen to exist for a fabrication process involving dual field oxide fabrication where the alignment marker is not diminished by the second field oxide layer and that facilitates checking for mask misalignment during subsequent masking operations.
Accordingly, a primary object of the present invention is to provide a mask set that produces an alignment mark that is accurate for subsequent fabrication process after undergoing a dual field oxide (FOX) fabrication process.